DETERMINATION OF THE LIPOPHILICITY OF FUSED-RING NITROGEN-HETEROCYCLES BY REVERSED-PHASE THIN-LAYER CHROMATOGRAPHY - THE EFFECT OF PH

The lipophilicity (R-M value) and specific hydrophobic surface area of 21 fused-ring nitrogen heterocycles has been determined by reversed-p hase thin-layer chromatography and the effect of different mobile-phas e additives such as acetic acid, sodium acetate, and lithium chloride on the retention has been studied. Regular retention behavior was obse rved for each solute, the R-M value decreasing linearly with increasin g concentration of acetonitrile in the mobile phase. Retention was red uced by acetic acid and increased by lithium chloride and sodium aceta te, acetic acid hating the strongest effect. The solute lipophilicity and specific hydrophobic surface area values for each mobile phase cou ld be intercorrelated, indicating the homologous character of the fuse d-ring nitrogen heterocycles under acidic, alkaline, and neutral condi tions. Spectral mapping showed clustering of the solutes according to the number of rings in the molecules, indicating that the ring structu res are planar and that they lie on the hydrophobic surface of the sta tionary phase.